The system design aspect of recent years has driven the requirements for special tools and practices to ensure high speed signaling quality, especially for backplanes and connector via arrays.
The speeds of signals used in the industry increases at about an average rate of twice every two years. As a result the rise time of signals used on backplanes and other printed circuit boards (PCBs) decreases and the bandwidth required to deliver those signals from point to point increases (doubles every two years). Transport data rates of 3.125 Gigabits per second (Gbps) are now commonplace in board-to-board applications. As data rates increase to 5, 6.25, or 10 Gbps each part of the channel must be examined to increase performance.
A channel includes plated through holes (PTHs), also called vias, that transport signals into interior layers of a multi-layer PCB as depicted in FIG. 1. PTHs are common to many device packages such a Ball Grid Arrays (BGAs) and other connector types. Typically, in a backplane system a signal path between a transmitter and receiver includes several vias or PTHs.
PTHs are fabricated by drilling a hole through a multi-layer PCB. As is known in the art, a multi-layer PCB includes conductive traces separated by dielectric layers. The hole is plated with a conductor, such as copper, and a pad is formed to connect the PTH to a particular one of the conductive traces. As depicted in FIG. 1, a PTH 10 may be utilized to conduct a signal from a capture pad 12 mounted on the surface 14 of the PCB to an internal trace 16. In this case, the PTH has a pad on the surface for connecting with the capture pad and another pad connected to the selected internal trace. The PTH is insulated from all other traces.
At high frequencies the PTH joining a surface pad to an interior trace will affect signal shaping. Depending on the frequency of the signal and the dimensions of the PTH, signal energy may be reflected from the PTH or converted into radiation thus causing loss of signal power and other undesirable side effects.
It is known that the frequencies where the PTH acts as a filter are determined by the unused portion of the hole, referred to as the resonant stub. In FIG. 1, the portion of the PTH that extends beyond the selected trace layer is the resonant stub 18. One technique for controlling the effects of the stub is to alter its size by a technique known as “back-drilling”, where plating is removed from the unused portion of the PTH by drilling from the back side of the PCB as depicted in FIG. 2. Back-drilling necessarily entails a tradeoff between manufacturing costs and electrical performance. Its effectiveness is limited by drilling depth accuracy and the increased cost of multi-depth drilling.
However, these simple back-drilling techniques (used to reduce the stub effect) in some cases are not adequate to guarantee high quality signaling at the speed of 2 Gbps and above.
Other known techniques exist that utilize more exotic technology, such as embedded passive or active filters in the PCB, to reduce reflections and shape signals. However, those techniques are expensive and, in most cases, not useful in mass production of PCBs.
The challenges in the field of high-frequency transmission continue to increase with demands for more and better techniques having greater simplicity and lower cost. Therefore, a need has arisen for a new system and method for controlling reflection and signal shaping caused by PTHs.